Earnings per Share (Tables) (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]

Basic earnings per share are based upon the weighted-average number of shares of common stock outstanding. Diluted earnings per share are based upon the weighted-average number of shares of common stock outstanding, plus all potentially dilutive securities that were assumed to be converted into common shares at the beginning of the period under the treasury stock method. Our potentially dilutive securities consist of restricted stock, restricted stock units, performance share awards and SSARs.

Share amounts in millions	Quarter Ended March 31,
	2018	2017
Denominator for basic earnings per share:
Weighted average common shares outstanding - basic	144.7	144.2
Effect of dilutive securities:
Dilutive effect of employee stock plans	2.0	1.7
Denominator for diluted earnings per share:
Weighted average shares outstanding - diluted	146.7	145.9